This invention relates generally to surface passivation of silicon devices, and more particularly the invention relates to passivation of silicon solar cells.
The silicon solar cell generates electrical charge when exposed to solar radiation. The radiation interacts with atoms of the silicon and forms electrons and holes which migrate to P and N doped regions in the silicon body and create voltage differentials between doped regions.
Typically, the silicon body is coated with a silicon oxide passivation layer. This layer may also serve as an anti-reflection layer to impinging radiation, or, alternatively, an additional anti-reflection coating can be applied over the passivation layer. Heretofore, a problem with oxide passivation layers has been a degradation of the surface recombination velocity at the silicon-silicon oxide interface when exposed to concentrated sunlight. This effect is primarily due to hot electrons being created by the ultraviolet end of the solar spectrum and being injected from the silicon into the oxide. One measure of the passivation stability is the emitter saturation current density, J.sub.o, which should be as low as possible.
Heretofore, attempts have been at improving the resistance to radiation damage in a silicon solar cell. Weinberg, et al., 4,608,452 and Wvsocki, 3,940,965 teach the counter-doping of the silicon body with lithium. However, this technique is structured to minimizing ionizing particle radiation damage of the silicon itself rather than ultraviolet radiation damage to the silicon/silicon oxide interface.
The present invention is directed to a method of improving the stability of a passivation layer on the surface of a silicon solar cell.